Carbon and graphite fibers and composites made therefrom are finding increasing uses in such diverse applications as lightweight aircraft and aerospace structures, automobile parts, and sporting equipment. Due to their high strength per weight ratio further added uses of these composites can be expected in the future.
Typically in the manufacture of carbon or graphite fibers a carbonaceous material is melted, spun into a thread or filament by conventional spinning techniques and thereafter the filament is converted to a carbon or graphite fiber. Conventionally the spun filament is stabilized, i.e., rendered infusible, through a heat treatment in an oxidizing atmosphere and thereafter heated to a higher temperature in an inert atmosphere to convert it into a carbon or graphite fiber.
The prior art discloses many different carbonaceous materials (sometimes called fiber precursors) that may be utilized to manufacture a carbon or graphite fiber. However, the two most significant commercial processes employ mesophase pitch or polyacrylonitrile. Through the use of such materials high strength graphite fibers can be produced.
In order for carbon or graphite fibers to be more widely accepted in commercial applications, improved, more economical fibers must be developed. Three significant manufacturing costs are the preparation of the feedstocks from which the fibers are produced, spinning of the fibers, and the cost of stabilizing the fibers and subsequently converting them to the end product.
In the manufacture of relatively expensive, structured high performance graphite fibers from mesophase pitch one of the most significant costs is the cost of producing the mesophase pitch. Most processes ordinarily require heating of a conventional pitch material at elevated temperatures over a period of several hours. For example, in Lewis et al U.S. Pat. No. 3,967,729, Singer U.S. Pat. No. 4,005,183, and Schulz U.S. Pat. No. 4,014,725, the preparation of the mesophase pitch requires that the initial feedstock be heated to an elevated temperature for a number of hours. Obviously such a process is time consuming and costly. Also care must be taken in heating for a specific time, as mesophase pitch can increase in viscosity rapidly, making it unsuitable for spinning.
The manufacture of graphite or carbon fibers from polyacrylonitrile also employes a relatively expensive feedstock in the process. It is generally thought that the overall cost of producing fibers from polyacrylonitrile is about equal to the cost of producing carbon or graphite fibers from mesophase pitch. With either process the final cost of the graphite fibers is currently $15 to $50 per pound.
Most of the commercial fibers produced from polyacrylonitrile or mesophase pitch have been fibers which have subsequently been converted to graphite fibers. Because the temperature of graphitization is higher than the temperature required to prepare a carbon fiber, graphite fibers are much more costly to produce than carbon fibers. However, certain mechanical properties of graphite fibers are generally superior to those of carbon fibers.
In the past attempts have been made to manufacture carbon fibers from pitch materials without first converting the pitch to the mesophase state. For various reasons these attempts have not been altogether successful and today there exists a need for a commercially economical process for manufacturing lower cost carbon fibers having intermediate mechanical properties from nonmesophase pitch materials, e.g. for asbestos replacement markets.
Various desirable and undesirable characteristics of the fiber precursor have been disclosed in the prior art. For example, Fuller et al U.S. Pat. No. 3,959,448 discloses that shorter stabilization times can be obtained if the softening point of coal tar pitch is increased. However, an attendant disadvantage has been recognized, namely that spinning fibers from coal tar pitch having a softening point of above 200.degree. C. is very difficult. See for example, Turner et al U.S. Pat. No. 3,767,741. Likewise, it has been recognized that handling carbon fibers made from pitch is relatively difficult. See for example, Kimura et al U.S. Pat. No. 3,639,953.
Otani U.S. Pat. No. 3,629,379 teaches the use of heat treatment at elevated temperature combined with high vacuum distillation, and heat treatment at elevated temperature combined with admixture of reactive species (peroxides, metal halides, etc.) to produce pitches suitable for melt or centrifugal spinning. The heat treatment step is about one hour, the distillation step is about three hours, and all operations are batch as opposed to continuous operation. Otani also teaches the desirability of reducing the aliphatic chain components to limit outgassing during carbonization, and the use of the above cited reactive species to reduce the stabilization time required to prepare the pitch fibers for carbonization.
Besides the softening point, other properties of the pitch material are also important. For example, the presence of impurities and particulates, molecular weight and molecular weight range, and aromaticity. Also, the chemical composition of the pitch material is important, expecially insofar as the stabilization of the fiber prior to carbonization is concerned. In fact, various additives and other techniques are taught in the prior art for addition to the pitch material in order to provide a pitch fiber that can be quickly and easily stabilized. See for example Barr et al European Patent Application No. 80400136.0 filed 28.01.80 Barr et al, Carbon Vol. 16 pp. 439-444 (Pergamon Press 1979), and Otani, U.S. Pat. No. 3,629,379.